feat(host-rules): Support `readOnly` request matching (#28562)

docs/usage/configuration-options.md

lib/config/options/index.ts

lib/modules/platform/github/index.ts

lib/types/host-rules.ts

lib/util/github/graphql/datasource-fetcher.ts

lib/util/host-rules.spec.ts

lib/util/host-rules.ts

lib/util/http/github.ts

lib/util/http/host-rules.ts

lib/util/http/index.ts

lib/util/http/types.ts

lib/workers/repository/update/pr/changelog/github/source.ts